ellipse in log-log

Community matplotlib-users
1

Dear list,

How is it possible to draw a nice ellipse in a log-log plot using patches.Ellipse ?
With matplotlib 0.91 I was able to to that using :
Ellipse((log10(100),log10(100)), width=100, height=100,alpha=0.5)

Now, it seems that something has changed in version 0.98 and I do not need to ad the log10
in Ellipse. However, instead of having a nice ellipse, I get a kind of patatoïd ?
See the example below.
How can I fix it ?

Thanks in advance,

yves

from numpy import *
import pylab as pt
from matplotlib.patches import Ellipse

ax = pt.gca()

e = Ellipse((100,100), width=100, height=100,alpha=0.5)
ax.add_artist(e)

pt.semilogx()
pt.semilogy()

pt.axis([1,1e3,1,1e3])

pt.show()

···

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--------------------------------------------oOO--(_)--OOo-------
  Yves Revaz
  Laboratory of Astrophysics EPFL
  Observatoire de Sauverny Tel : ++ 41 22 379 24 28
  51. Ch. des Maillettes Fax : ++ 41 22 379 22 05
  1290 Sauverny e-mail : Yves.Revaz@...2003...
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2

In 0.98 ellipse is drawn using bezier curves, which do not scale correctly in log-scaled plots.

As an alternative, you can use a RegularPolygon with a high number of vertices (this is exactly what 0.91 did).

Mike

Yves Revaz wrote:

···

Dear list,

How is it possible to draw a nice ellipse in a log-log plot using patches.Ellipse ?
With matplotlib 0.91 I was able to to that using :
Ellipse((log10(100),log10(100)), width=100, height=100,alpha=0.5)

Now, it seems that something has changed in version 0.98 and I do not need to ad the log10
in Ellipse. However, instead of having a nice ellipse, I get a kind of patato�d ?
See the example below.
How can I fix it ?

Thanks in advance,

yves

from numpy import *
import pylab as pt
from matplotlib.patches import Ellipse

ax = pt.gca()

e = Ellipse((100,100), width=100, height=100,alpha=0.5)
ax.add_artist(e)

pt.semilogx()
pt.semilogy()

pt.axis([1,1e3,1,1e3])

pt.show()

--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

3

Here is some example code which compares the polygonal ellipse with
the spline version to get you started if you decide to go with the
polygonal appromximation Michal suggest:

import numpy as npy
from matplotlib import patches
from pylab import figure, show

xcenter, ycenter = 0.38, 0.52
#xcenter, ycenter = 0., 0.
width, height = 1e-1, 3e-1
angle = -30

theta = npy.arange(0.0, 360.0, 1.0)*npy.pi/180.0
x = width/2. * npy.cos(theta)
y = height/2. * npy.sin(theta)

rtheta = angle*npy.pi/180.
R = npy.array([
    [npy.cos(rtheta), -npy.sin(rtheta)],
    [npy.sin(rtheta), npy.cos(rtheta)],
    ])

x, y = npy.dot(R, npy.array([x, y]))
x += xcenter
y += ycenter

fig = figure()
ax = fig.add_subplot(211, aspect='auto')
ax.fill(x, y, alpha=0.2, facecolor='yellow', edgecolor='yellow',
linewidth=1, zorder=1)

e1 = patches.Arc((xcenter, ycenter), width, height,
             angle=angle, linewidth=2, fill=False, zorder=2)

ax.add_patch(e1)

ax = fig.add_subplot(212, aspect='equal')
ax.fill(x, y, alpha=0.2, facecolor='green', edgecolor='green', zorder=1)
e2 = patches.Arc((xcenter, ycenter), width, height,
             angle=angle, linewidth=2, fill=False, zorder=2)

ax.add_patch(e2)

#fig.savefig('ellipse_compare.png')
fig.savefig('ellipse_compare')

show()

···

On Mon, Oct 6, 2008 at 7:15 AM, Michael Droettboom <mdroe@...86...> wrote:

In 0.98 ellipse is drawn using bezier curves, which do not scale
correctly in log-scaled plots.

As an alternative, you can use a RegularPolygon with a high number of
vertices (this is exactly what 0.91 did).